The present invention relates to a molten carbonate fuel cell (MCFC) anode and a method for manufacturing the same, and more particularly, to an MCFC anode comprising pure nickel (Ni) powder and Ni-coated alumina powder and a method for manufacturing the same, for improving creep resistance and performance of a nickel (Ni) electrode used as the MCFC anode.
A fuel cell is a new electricity generating system for directly converting the energy produced by electrochemical reaction of a fuel gas and an oxidizing gas into electric energy. A fuel cell is similar to a general cell in that it is composed of two electrodes and an electrolyte, but different in that it is provided continuously with fuel and an oxidizing agent as a reactant. A fuel cell is under careful examination for use as power generating equipment, a power source for space stations, a power source for unmanned facilities at sea or along costal areas, a power source for fixed or mobile radios, a power source for automobiles or a power source for household electrical appliances.
Fuel cells are divided into a molten carbonate electrolytic fuel cell which is operated at a high temperature in the range of about 500xc2x0 C. to about 700xc2x0 C., a phosphate electrolytic fuel cell which is operated around 200xc2x0 C., an alkaline electrolytic fuel cell which is operated at room temperature to about 100xc2x0 C. or below and a solid electrolytic fuel call which is operated at a high temperature of 1,000xc2x0 C. or above.
A molten carbonate fuel cell is constituted by a porous Ni anode, a Li-doped porous Ni oxide cathode and Lixe2x80x94Al matrix which is filled with lithium and potassium carbonate as the electrolytes. The electrolytes are molten-ionized at about 500xc2x0 C., and the carbonate ion generated therefrom carries charges between the electrodes. Hydrogen is consumed in the anode area to produce water, carbon dioxide and electrons. The electrons flow to the cathode via an external circuit to produce the desired current flow.
Porous anodes of nickel, cobalt and copper-base are typically used in MCFCs. As mentioned above, since MCFCs are operated at a high temperature of about 650xc2x0 C., pressure is also applied thereon in order to improve the contact between electrodes and electrolyte matrix, and various layers are stacked to produce the pressure by the load of the stack itself, a creep deformation of the anode is generated. Creep deformation of electrodes is irrevocable and occurs by a combination of at least three different mechanisms of particle rearrangement, sintering and dislocation movement. In other words, performance of electrodes is lowered in various respects, i.e., comminuted pores are reduced due to non-uniform creeps of the respective parts of the anode, thereby reducing the reactive area of the electrode, the contact between electrodes and electrolyte elements becomes poor and the leakage of fuel gas may occur.
Therefore, various methods have been attempted to prevent such an undesirable creep deformation in MCFC anodes. One of the methods is to manufacture anodes by adding Cr or Al to Ni. For example, if a creep test is performed for 100 hours under the conditions of 100 psi and 650xc2x0 C., the creep rate is about 30% to 50% in the pure porous Ni electrode. In the case of Nixe2x80x94LiAlO2 obtained by adding LiAlO2 to the Ni electrode, the creep rate is about 14% to 35%. Likewise, in the cases of Nixe2x80x94Cr (10%) to which lot Cr is added and Nixe2x80x94Al (10%) to which 10% Al is added, the creep rates are lowered to 5% to 10% and about 2%, respectively. However, a satisfactory performance in operating MCFCs for a long time (40,000 hours) by the development of large capacity of MCFCs has not been achieved yet.
Although Nixe2x80x94Cr (10%) anodes have been widely used so far, since the price of Cr is high, it is under consideration to add Al to Ni. If Al is added to Ni, the creep rate is lowered to 2% or less and the production cost is lowered since Al is cheaper than Cr.
Nixe2x80x94Al anodes or Nixe2x80x94(Al, Cr) anodes obtained by adding Al or Cr to Ni are formed by the same procedure as used for manufacturing the prior anode by forming green sheets by a casting method after forming an alloy powder of Ni and additive metals. However, it is difficult to form a comminuted alloy powder of Ni and metals.
One of the methods developed for preventing creep deformation of a porous anode structure is to internally oxidize alloy metals typically used in the base metal-alloy metal composition.
For example, U.S. Pat. No. 4,314,777 discloses a method for internally oxidizing alloy metal by a heat treatment of the blend powder of an alloy powder and an oxidant base metal, but it is not suitable for use as a porous anode structure because of an end product having high density.
U.S. Pat. No. 4,714,586, which discloses a method for forming dimensionally stable Nixe2x80x94Cr anodes by internally oxidizing the alloy metal at is high water vapor pressures, is limited to the formation of Nixe2x80x94Cr anodes.
To solve the aforementioned problems, U.S. Pat. No. 4,999,155 discloses a method for manufacturing MCFC anodes with improved creep resistance property. That is to say, base metal power and alloy metal powder are blended with a binder and solvent and then cast, dried and sintered to form a porous anode structure. Then, oxidant particles are formed internally by internally oxidizing the alloy metal under conditions in which the base metal is reduced and the alloy metal is oxidized.
Another method disclosed in U.S. Pat. No. 4,999,155 is forming an alloy including a base metal and an alloy metal, oxidizing the surface of the alloy by a heat treatment and simultaneously sintering and internally oxidizing the oxidized alloy. Here, the introduction of the alloy metal to the base metal is achieved by tape casting and sintering the mixture of the base metal powder and the alloy metal powder and then diffusing the alloy metal to the base metal. Alternatively, a vapor deposition or a pack cementation is performed after the base metal powder is tape-cast and sintered into a porous structure.
According to this pack cementation method, Ni metal powder is first mixed with a binder and solvent, is cast and is then dried and sintered to form a porous Ni sintered body. Then, the porous nickel sintered body is embedded in a pack consisting of alloy metal powder such as Al, an activator salt and an inert filler, and is heated to about 900xc2x0 C. in an atmosphere of 10% H2/90% N2, performing a pack cementation and thereby forming Nixe2x80x94Al alloy. Next, the structure is then internally oxidized at temperatures of 600xc2x0 C. to 800xc2x0 C. in a humidified atmosphere having a pH2O/pH2 greater than 20. According to this method, although the excellent effect in decreasing creep deformation can be obtained, since various complicated procedures such as drying, sintering, pack cementation and internal oxidization should be performed after forming a green sheet with a Ni base metal, practical problems still remain.
As described above, since an anode based on Ni-metal alloy powder and the manufacturing thereof was mainly employed, problems caused by an alloy system could not be completely solved.
In consideration of various problems as mentioned above, an object of the present invention is to provide an MCFC anode for decreasing creep deformation of an MCFC anode and enhancing the performance of an electrode.
It is another object of the present invention to provide a method for manufacturing an MCFC anode by adding a pure Ni powder and a Ni-coated alumina powder where the surface of the ceramic alumina powder is coated with Ni.
To accomplish the above and other objects of the present invention, there is provided an MCFC anode comprising a pure Ni powder and Ni-coated alumina powder.
Here, it is preferable to regulate the composition ratio of pure Ni powder and Ni-coated alumina powder such that the content of Al is 4-6 wt. % based on the total weight thereof.
To accomplish another object of the present invention, there is provided a method for forming an MCFC anode according to the present invention comprising the steps of: forming a Ni coating solution for coating an alumina powder by mixing nickel acetate and ethanol in a predetermined ratio to form a mixture, adding distilled water to the mixture in a predetermined ratio to form a resultant, and refluxing the resultant; coating a pre-treated surface of the alumina powder with the Ni coating solution to form Ni-coated alumina powder; mixing a pure Ni powder and the Ni-coated alumina powder to form a mixture of pure Ni powder and Ni-coated alumina powder, and then forming a green sheet of an electrode from the mixture of pure Ni powder and Ni-coated alumina powder; and drying and sintering the electrode in a reducing atmosphere.